The present invention relates to a two-part room temperature vulcanizable silicone composition and more specifically the present invention relates to a two-part room temperature vulcanizable silicone rubber composition which in the cured state is extremely hydrolytically stable and has high resistance to reversion.
Two-part room temperature vulcanizable rubber compositions are well known compositions in the prior art. Such compositions generally comprise as ingredients a silanol-stopped diorganopolysiloxane and a filler which comprises one-part of the two-component composition. The second part of the two-component composition usually comprises an alkyl silicate or a partial hydrolysis product of an alkyl silicate as the cross-linking agent combined with a catalyst which traditionally has been the metallic salt of an organic monocarboxylic or dicarboxylic acid in which the metal ion is selected from the class consisting of lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth and manganese.
One trouble with this type of catalyst which has been experienced in practice is that the catalyst is not adequately soluble and in most cases is found to be insoluble in the alkyl silicate of the second component. As a result when the two components are mixed together to cure the composition to form a cured two-part room temperature vulcanizable silicone rubber composition, it is found the insoluble catalyst does not readily disperse throughout the entire composition and is only erratically dispersed through the total composition. As a result of the inadequate dispersion, the two-part room temperature vulcanizable silicone rubber composition does not cure uniformly. In fact, there may be experienced weak spots in the cure even after the two-part room temperature vulcanizable silicon rubber composition has been allowed to lie for a substantial period of time so as to allow it to cure. The solution to this problem has been met by, in most cases, urging the applicators of the two-part RTV system (RTV standing for room temperature vulcanizable silicone rubber composition) to mix the two components one into the other very thoroughly. However, as can be envisioned, this is not only time consuming but extremely difficult to carry out in practice.
Another problem with the utilization of the metal salts of a monocarboxylic acid or dicarboxylic acid catalyst is that after the two-part RTV system is cured and especially in warm and humid climates, water vapor or moisture might react with the catalyst which was present in the cured two-part RTV system and result in the liberation of acids which over a period of time will degrade the cured two-part RTV system to some extent. Although the two-part RTV system even if degraded in this manner, will retain most of its physical properties, nevertheless, there resulted the phenomenon of what is known as "chalking", that is, the surface RTV layer would degrade somewhat and present an unsightly appearance to the eye. This reversion or degradation of the RTV system was especially found to be evident when the traditional two-part RTV systems were cured to form roof coatings in humid climates.
Accordingly, it would be highly desirable to provide a catalyst system for a two-part RTV system which catalyst would be easily soluble in the alkyl silicate cross-linking agent in traditional two-part RTV systems as well as the linear silanol-stopped diorganopolysiloxane material and also to provide a catalyst which even in hot and humid climates would not release or degrade to form acids which might attack and cause reversion of the cured two-part RTV system.
Therefore, it is one object of the present invention to provide a novel catalyst for a two-part RTV system which catalyst would be soluble in the other fluids which are utilized to produce the uncured RTV system.
It is an additional object of the present invention to provide a catalyst for a two-part RTV system which catalyst even in hot and humid climates will not release any acids which might degrade the cured RTV polymer.
It is an additional object of the present invention to provide a novel catalyst for two-part RTV systems which can easily be incorporated into a two-part self-bonding RTV system so as to produce from such two-part RTV systems coatings and films of exceptional hydrolytic stability.
These and other objects of the present invention are accomplished in accordance with the disclosure set forth herein below.